1. Chap. 1 - Part I Composition of the Atmosphere
WX 201
Dr. Chris Herbster

2. Outline Meteorology Defined The atmosphere as a gas
Permanent and Variable Gases
Influence by planet size and distance from the Sun on atmospheric composition
Composition of Earth’s atmosphere
Comparisons with Mars and Venus
Unique features of Earth’s atmosphere compared to the other planets

3. What is Meteorology?
The study of the atmosphere and the processes that cause “weather” (cloud formation, lightning, wind movement)
Weather deals with the short term state of the atmosphere
Climate deals with the long-term patterns
More than simple long-term averages
Involves complex interactions and variability

4. Thickness of the Atmosphere
Approximately 80% of the atmosphere occurs in the lowest 20km above the Earth.
Radius of the Earth is over 6,000 km
Atmosphere is a thin shell covering the Earth.

5. But what is the atmosphere?
Comprised of a mixture of invisible permanent and variable gases as well as suspended microscopic particles (both liquid and solid)
Permanent Gases – Form a constant proportion of the total atmospheric mass
Variable Gases – Distribution and concentration varies in space and time
Aerosols – Suspended particles and liquid droplets (excluding cloud droplets)

6. Composition of Earth’s Atmosphere
Important gases in the Earth’s Atmosphere
(Note: Influence not necessarily proportional to % by volume!)
Water vapor: extremely important due to its being found in all three phases in Earth’s atmosphere, latent heat release, etc.
Carbon dioxide: important gas due to its role as “greenhouse gas” (not totally correct term as we’ll see later)
Ozone: dual importance (pollutant in troposphere, but very important absorber of UV in stratosphere)
CFCs: Influence on ozone hole

7. Permanent Gases 78% Nitrogen (N2) 21% Oxygen (O2) <1% Argon (Ar)
Relative percentages of the permanent gases remain constant up to km high (~ 60 miles!)
This layer is referred to as the Homosphere (implies gases are relatively homogeneous)

8. Homosphere and Heterosphere
Homosphere: Turbulent mixing causes atmospheric composition to be fairly homogenous from surface to ~ km (i.e., 78% N2, 21% O2)
Heterosphere: Above ~80-100km, much lower density, molecular collisions much less, heavier molecules (e.g., N2, O2) settle lower, lighter molecules (e.g., H2, He) float to top

9. Variable Gases in the Earth’s Atmosphere
VARIABLE gases in the atmosphere and typical percentage
values (by volume):
Water vapor (H2O) 0 to 4%
Carbon Dioxide (CO2) %
Methane(CH4)         %
Ozone(O3)            %
(Note that water vapor is the third most common molecule in Earth’s atmosphere after nitrogen and oxygen)

10. Variable Gases - Water Vapor
Water vapor is invisible – don’t confuse it with cloud droplets
Less than 0.25% of total atmosphere
Surface percentages vary between <<1% in desserts to 4% in tropics
Typical mid-latitude value is about 1-2%
Some satellites sensors can detect actual water vapor in atmosphere
Water Vapor Image
Visible Image

11. Variable Gases - Carbon Dioxide (CO2)
Small percentage of total atmosphere (380 ppm)
But, very important green house gas
How many have seen the movie “An Inconvenient Truth” or read the book?
Mauna Loa Observatory CO2 trace
(annual variations embedded in the long-term record)

12. Atmospheric CO2 cycle. Global climate models used to examine greenhouse warming must be able to account for multiple, complex processes in atmosphere, over land, and in ocean.
Earth’s greenhouse gases contribute to a ~30C warmer surface temperature than would otherwise exist. More on this phenomenon in Ch. 2.

13. Variable Gases – Ozone (O3)
Near the surface, ozone concentrations about ppm
In the upper atmosphere ozone concentration can reach ~15 ppm
Upper atmospheric ozone is vital to blocking harmful radiation
Ozone near the surface, however, harmful to life
Chlorofluorocarbons (CFCs) are believed to be depleting upper atmospheric ozone
Satellite images showing depletion of ozone.

14. Variable Gases – Methane (CH4)
Concentrations of about 1.7 ppm
Extremely potent green house gas - 21 times more powerful by weight than carbon dioxide
Has varied cyclically on a 23,000 year cycle
Pattern broken in past 5,000 years with unexpected increase – more abundant now than in last 400,000 years
Increase attributed to agriculture, bio-mass burning, fossil fuel extraction, some industry and ruminant out-gassing (cow/sheep burps)
Methane growth and sources (From EPA)

15. Aerosols (or Particulates)
Small (or “tiny”) solid particles or liquid droplets (excluding clouds and rain)
Aerosols can be man-made (anthropogenic) or naturally occurring (like ocean salt, dust, plant emissions)
Aerosols are not synonymous with pollution
Some aerosols are very beneficial and, in fact, are required for precipitation processes to occur.

16. What Determines Atmospheric Composition?
Composition of gases on a planet is determined largely by how easily gases can escape to space
Also depends on the existence of life or geologic processes
For a gas to escape to space, it must reach its “escape velocity.”
Escape velocity is the speed required to overcome the gravitational pull of the planet
Molecular velocity is determined by the gas temperature (or average kinetic energy)

17. Escape Velocity Gas is made up of free molecules in constant motion.
Speed of the gas molecules is determined by the temperature
Temperature determined largely by proximity to the Sun
Escape velocity depends on the gases’ molecular weight and the planets size
Lighter molecules require less speed to escape
Larger planets have stronger gravitational pull

18. Relative Planet Size and Distance from Sun
Size comparison of planets – larger planets have stronger gravitational pull
Planets closer to the Sun receive more radiant energy

19. The required “escape velocity” is determined planet size
Temperature of gas determined
by distance from sun.
Molecular speed determined by molecular weight and temperature
Gas lines above the planet
will escape to space.
Gas lines below the planet will
remain in the atmosphere.
i.e. Earth will lose hydrogen
but hold water. Mars will
lose water but hold
carbon dioxide.

20. Earth’s Early Atmosphere
5 Billion years ago when Earth formed, atmosphere consisted of mostly H2 , He as well as some NH3 , and CH4.
Free H2 and He molecules have low molecular weight (so move very fast), and were able to escape Earth’s gravitational pull.
Volcanoes spewed large amounts of H2O, CO2 as well as lesser amounts of N2 (outgassing)
Clouds rained forming oceans, which dissolved much of CO2 locking it in sedimentary rocks through chemical and biological processes (e.g., seashell formation) allowing concentrations of N2 to increase.
O2 increased through phododissociation of H2O into H2 and O2—the H2 escaped.
Life formed, plants grew adding additional O2 through photosynthesis leading to today’s atmosphere.

21. Unique Features of Earth’s Atmosphere
Atmospheric composition – high Oxygen content, low Carbon Dioxide content.
Greenhouse gases contribute to livable surface temperatures
Most important greenhouse gas is water vapor!
Without an atmosphere, Earth’s surface temp would only be approximately 0°F!
Water in all three phases: solid, liquid, gas.
Patchy cloud fields – extensive up and down convective motions in atmosphere.
Circular motions with storms.

22. Comparison with Venus
Composition of Venus Atmosphere: 96% CO2, 3% N2 (compare to Earth—.04% CO2, 78% N2)
Pressure at surface: 90,000 mbar (by comparison, Earth’s mean sea-level pressure is approximately 1,013 mbar — Venus’ surface pressure is 90x greater!)
Temperature at surface: ~ 900oF (by comparison, Earth’s mean sfc temperature is about 59oF)
Extreme atmospheric pressures on Venus due large amount of gaseous CO2.
No mechanisms to remove CO2 from atmosphere (e.g., photosynthesis, dissolution in water).

23. Earth and Venus nearly same size – velocity required to escape gravitational pull similar for both.

24. Why the drastic difference?
Venus is closer to Sun
Warmer temperatures prevented liquid water from forming.
With no liquid water, no means to dissolve the carbon dioxide.
Result is a rich carbon dioxide atmosphere.

25. Earth and Venus CO2 and N2
Earth actually has more CO2 than Venus (as fraction of total planet mass).
Earth and Venus have similar amounts of N2.
CO2 is 96% of Venus atmosphere and only .04% of Earth’s.
Venus has CO2 in atmosphere, while Earth has CO2 in limestone.

26. Mars About half the size of the earth (less gravity)
Atmosphere primarily CO2 -- too heavy to escape gravitational pull
Surface pressure 1/100 of earth’s (~10 mbar)
Average surface T~213K (-76F)
Temperature between equator and poles 130C.
Temperature change of 60C between day and night (low thermal inertia)
Ice caps at poles composed of frozen CO2
Small size of planet allowed most of atmosphere to escape

27. Weather on Earth in relation to orbital characteristics
Rotation once per 24 hrs.
Primary weather systems are moving storms with clouds, circular winds, and precipitation

28. Weather on Venus in relation to orbital characteristics
Rotation once per 243 (earth) days (Venus day is longer than year)
Thick atmosphere of CO2 causes greenhouse “pressure cooker.” Surface temperatures ~ 900 deg. F.
Uniform temperatures all over globe, little surface winds but strong upper level winds.

29. Weather on Mars in relation to orbital characteristics
Rotation once per 24.6 hours.
Surface temperature from
–200 to +80 F.
Has frequent dust storms.
Has polar caps of CO2 and H2O.
Seasonal change causes caps to melt
and reform.
Has very few clouds.

30. Summary
Composition of gases on a planet is a function of the planet size (strength of gravity holding gases onto the planet), planet temperature, and life
Primary permanent gases on Earth are Nitrogen, Oxygen, Argon
Variable gases include Water Vapor, Carbon Dioxide, Ozone, Methane, CFCs, etc.
The importance of variable trace gases is not always proportional to the amount.

31. Summary (cont.)
Water vapor is the most important greenhouse gas, others include Carbon Dioxide, Methane and Ozone
Gases on other planets are quite different from Earth’s because of differing planet characteristics (Venus & Mars have primarily CO2 atmospheres)
Weather on Earth different from weather on other planets because of gas composition, planet size, oceans and planet rotation speed

32. Chap. 1 - Part II Fundamental Quantities ~ Vertical Structure of the Atmosphere ~ Weather Basics
WX 201
Dr. Chris Herbster

33. Outline Fundamental physical quantities covered in this course
Atmospheric state variables
Density, Pressure, temperature
Structure of the atmosphere
Troposphere
Stratosphere
Mesosphere
Thermosphere
Importance of the stratosphere and thermosphere

34. Fundamental Physical Quantities Units of Measure Needed for this Course
Basic Quantities
Quantity Symbol SI Unit Equivalent Units
Length L Meter (m) 1 m ≈ 3.28 ft
Mass m Kilogram (kg) 1 kg ≈ lb
Time t Second (s) 60 s = 1 min
Temperature T Kelvin (K) K ≈ 0°C = 32°F
Derived Quantities
Area A = L2 Sq meter (m2) 1 m2 ≈ ft2
Volume V = L3 Cu meter (m3) 1 m3 ≈ 35.3 ft3
Density r=m/V Kg/m3 1 kg/m3 ≈ lb/ft3
Velocity V = L/t m/s 1 m/s ≈ 2.24 mph ≈ 1.94 kt
Acceleration a = V/t m/s2
Force F = m·a Newton (N) 1 N = 1 kg·m/s2
Weight Wt = m·go Newton (N) 1 N ≈ lb; go ≈ 9.8 m/s2
Mass (m) Amount of matter in an object.
Length (L) A measurement of distance.
Time (t) A period over which an action takes place

35. Fundamental Physical Quantities (cont.)
Derived Quantities (cont.)
Quantity Symbol SI Unit Equivalent Units
Pressure p = F/a Pascal (Pa)* 1Pa = 10-2 mb = 100 N/m2
1hPa = 1 mb
1013 hPa ≈ in Hg
Energy/Heat/ E = F·L Joule (J) 1 J = 1 N-m
Work cal ≈ J
(note: 1 cal is the amount of heat needed to raise 1 g of water 1 K)
Power P = E/t Watt (W) 1 W = 1 J/s
* Meteorologists tend to use milli-bars (mb), which are identical equivalent to hecto-Pascals (hPa). We’ll use mb and hPa interchangeably in this course.
Some Useful Conversions
1 knot (kt) ≈ 1.15 mph ≈ m/s
1 inch Mercury (in Hg) ≈ mb
Centigrade (Celsius) to Kelvin: Add to deg C
Centigrade to Fahrenheit: Multiply by 1.8, then add 32
Fahrenheit to Centigrade: Subtract 32, then multiply by 5/9

36. Scientific Notation Prefix # of Base Units Scientific Notation
Terra (T)
Giga (G)
Mega(M)
Kilo (k)
1,000,000,000,000
1,000,000,000
1,000,000
1,000
(1012)
(109)
(106)
(10³)
Hecto (h)
100
(10²)
Deca (da) 10
(10¹)
Base 1
(10°)
Deci (d)
1/10
(10 ‾ ¹)
Centi (c)
1/100
(10 ‾ ²)
Milli (m)
1/1,000
(10 ‾ ³)
Micro (µ)
Nano (n)
1/1,000,000
1/1,000,000,000
(10‾6)
(10-9)
Unit Prefixes are used to determine very large numbers from very small numbers. Placed ahead of the base units, (i.e. Kilogram, decameter, millisecond).

37. Scientific Measurements
Significant Digits:
Nearest reportable values for common measurements
Upper Air Wind Speeds: 5 Knots
Surface Wind Speeds: Whole Knot
Upper Air Pressure: Whole Millibar (mb)
Surface Pressure: 1/10 (.1) mb
Skew-T Temperatures: 1/10 (.1) Degree
Temperatures: Whole Degree
Relative Humidity: Whole Percent
Upper Air Heights: Decameter
Important concept when dealing with numerical values in weather.

38. Atmospheric State Variables
State variables include:
Pressure
Temperature
Density
State variables are related to one another by the Ideal Gas Law (IDL)
IDL often referred to as the “Equation of State”
The state variables will be detailed throughout the course.

39. State Variables Pressure
Air is mostly made up of free molecules in constant motion (gases).
Air molecules have mass.
You can feel the mass of the air when the wind is blowing hard.
Weight (a vertical force) = Mass x Gravity
Air has mass therefore weight; pressure (weight/area) is measured by a barometer.

40. Surface Pressure
The pressure at the surface is caused by the weight of all the air molecules in the column above the surface.
Add more air molecules to the column and the pressure goes up. (High Pressure areas)
Take away air molecules from the column and the pressure goes down. (Low Pressure areas)

41. Pressure as Measured by Barometer
Weight of mercury in column equals weight of atmosphere
Average sea level pressure is:
14.7 pounds per square inch,
760 mm or 29.92” mercury or mb

42. State Variables Density
Air density is the mass of the air divided by the volume of measurement.
As one goes higher in the atmosphere the number of molecules in a given volume decreases, so like pressure, density also decreases monotonically with height.
Since don’t have as many molecules on top of you, the air pressure also decreases with height.

43. Density and Pressure with Height
Because of compression, the atmosphere is more dense near the surface.
Density decreases with altitude

44. State Variables Temperature
Air molecules are moving all around us, bouncing off each other and us.
When the air molecules have greater kinetic energy (energy of motion), they are moving faster.
The temperature of the air molecules is a measure of the average speed of the molecules per standard volume

45. Temperature Scales
K = °C
F = 9/5°C + 32
C = 5/9(°F – 32)

46. Temperature Change w/Altitude
As a parcel of air rises, it expands due to lower pressure.
Work done by molecules to expand causes temperature to decrease (cools)
As air sinks, the parcel experiences compression due to higher pressure
Air molecules have work done on them, temperature increases (warms)

47. Air Temperature Change w/ Changes in Parcel Altitude
Rising  Expansion  Cooling
Sinking  Compression  Warming

48. Relating State Variables: “Equation of State” or “Ideal Gas Law”
Temperature, pressure and density related
Pressure = density*gas constant*temperature
P = ρRT
If the pressure decreases, the density will decrease for constant Temp.
If the pressure decreases, the temperature will decrease for constant density, etc.
It is possible for all three state variables to change at the same time!
More in later chapters

49. Vertical Structure of the Atmosphere
Vertical Structure of the Atmosphere commonly broken into layers
Layers are most often defined by the vertical change of temperature within the layer since this is related to the presence of vertical motions (or lack of) in the layer

50. Temperature Layers of the Atmosphere: Troposphere
Lower part of the atmosphere
Energy source is heating of the earth’s surface by the sun.
Temperature generally decreases with height.
Air circulations (weather) take place mainly here.
Troposphere goes from surface to about 30,000 ft. (10 km).

51. Temperature Layers of the Atmosphere: Stratosphere
Sun’s ultraviolet light is absorbed by ozone, heating the air.
Heating causes increase of temperature with height.
Boundary between troposphere and stratosphere is the tropopause.
Stratosphere goes from about 10 to 50 km above the surface.

52. Temperature Layers of the Atmosphere: Mesosphere
Above 50 km, very little ozone, so no solar heating
Air continues to cool with height in mesosphere
Mesosphere extends from about 50 km to 90 km above the surface

53. Temperature Layers of the Atmosphere: Thermosphere
Above 90 km, residual atmospheric molecules absorb solar wind of nuclear particles, x-rays and gamma rays.
Absorbed energy causes increase of temperature with height.
Air molecules are moving fast, but the pressure is very low at these heights.

54. Importance of Stratosphere, Mesosphere and Thermosphere
Solar nuclear particles, x-rays, gamma rays, and ultraviolet light can damage living cells.
Thermosphere, mesosphere and stratosphere shield life on Earth from these damaging rays.

55. Weather Basics Atmospheric Pressure
Horizontal pressure differences cause the wind
Air tends to blow, at an angle, from high pressure to low pressure near the surface
Effect of rotating planet is that wind blows along a near constant pressure trajectory when friction is minimal
Pressure is identified on weather maps using isobars (iso = constant, bar = pressure).

56. Weather Basics Atmospheric Temperature
Areas separating colder and warmer air on a weather map are represented by fronts
Cold Fronts (blue – pointed barbs) indicate the movement of a cold air mass into a warmer region
Warm Fronts (red – rounded barbs) indicate movement a warm air mass into a colder region
Cold Front
Warm Front

57. Weather Basics Atmospheric Humidity
Relative Humidity provides a measure of the amount of water vapor in the air relative the maximum possible for a given temperature
Dew Point Temperature is the temperature the air must be cooled to for condensation to occur.
Much more on these concepts in later chapters

58. Weather Basics Weather Map

59. Weather Basics Station Plot

60. Summary
Atmospheric pressure caused by weight of column of air above you.
Pressure changes because of adding or taking away air from the column.
Temperature is a measure of the average speed of the molecules per standard volume.
Density is the mass per volume
Pressure, Temperature, and Density all related by the Ideal Gas Law (a.k.a. the Equation of State)

61. Summary (cont.)
Temperature decreases with height unless energy is added.
Troposphere temperature decreases with height.
Stratosphere temperature increases with height because of ozone absorption of dangerous UV radiation
Mesosphere temperature decreases with height
Thermosphere temperature increases with height because of absorption of solar particles, x-rays and gamma rays.
Atmospheric composition remains fairly homogeneous up to ~ km

62. A little more on pressure
Net Forces=0
If all sides of an object are exposed to the air pressure, the net forces will cancel each other out.
Pressure outside balloon equals the pressure inside plus the tension
of the balloon, so no air moves.

63. Balance of Forces Not Equal to Zero
Upward force of molecules balanced by downward force of weight of molecules above.
Sideways force of molecules balanced by sideways force of molecules next to the air parcel.
If some of the surrounding air is removed, then the molecules will be forced into the lower pressure region, causing “wind”.

64. Pressure Differences in the Horizontal
Fluids will flow from regions of high pressure to low pressure.
Consider the apparatus below
The pressure at the surface is proportional to the weight (or height) of the fluid above.
The fluid will flow from left to right until the surface pressures on both sides are equal.
High Pressure
Low Pressure

65. Pressure Differences in the Horizontal
Now consider the atmosphere
If pressure is higher in one location than another at same elevation, gas molecules will move from high pressure towards lower pressure.
In absence of influence by Earth’s rotation
Movement of gas molecules is the wind.
Pressure differences cause wind. (will cover in more detail in chapter 9)

66. Pressure Differences in the Vertical
Near sea level, pressure decreases about 1 mb for every 10 meter (33 ft) increase with height.
At 700 mb, 30% of atmosphere is below you and 70% is still above you.
700 mb = 3 km = 10,000 ft. (approximately)
At 500 mb, half the atmosphere is below you.
500 mb = 5.5 km = 18,000 ft (approximately)
250mb = 10.5 km = 34,400 ft. (approximately)
From previous slide, we saw that air will flow from higher
to lower pressure. Why doesn’t the air flow straight up given
that the pressure decreases rapidly with height?

67. Pressure in the Vertical
Pressure decreases “monotonically” with height.
Pressure always decreases with increasing height.
Often convenient to use pressure instead of height as our vertical coordinate.
Meteorologists frequently refer to the temperature, moisture and winds at standard pressure levels, e.g., 925, 850, 700, 500, 300, 250mb pressure levels.

68. Pressure Altimeter
Change of pressure with height can be used to measure altitude of aircraft.

69. The mysterious cockpit picture from the ERAU tornado – confirmed and re-confirmed by our faculty
Airspeed indicates 120 kt
Altimeter indicates 2000’
(equiv. to a 70 mb pressure drop!)
These readings would confirm the NWS
estimate of F2 damage from this tornado